Late Infantile Neuronal Ceroid Lipofuscinosis (LINCL) is a childhood-onset autosomal recessive neurodegenerative disease. This disorder, a lysosomal storage disease (LSD), is caused by mutations in CLN2, the gene that encodes the lysosomal hydrolase tripeptidyl peptidase I (TPP1). TPP1 deficiency causes epileptic seizures, vision impairment, cognitive and motor dysfunction, and is invariably fatal by late childhood. Currently, there is no treatment or cure for this devastating disease. The major barrier to treatment is the delivery of bioactive TPP1 enzyme to the central nervous system, a goal hindered by the selectivity of the blood-brain-barrier (BBB). In recent studies, our lab has engineered a brain-homing adeno-associated virus (AAV) gene delivery vector. Brain tropism of the virus is conferred by a peptide, GMNAFRA, which we identified by in vivo phage display panning in a mouse mode of LINCL. The GMN peptide has affinity for the luminal surface of brain vascular endothelial cells, which are the primary component of the BBB. When delivered peripherally, this peptide-modified virus (GMN-AAV) travels to, and transduces brain endothelia. Significantly, delivery of GMN-AAV encoding CLN2 in LINCL mice restored TPP1 expression in the brain and corrected neuropathological defects. Thus, this novel gene therapy approach bypasses the BBB by engineering endothelial cells to secrete recombinant enzyme directly into the underlying neuropil. In order to advance this exciting and promising gene therapy approach to LINCL patients, elucidation of the identity of the brain endothelial receptor molecule that interacts with the GMN peptide is required. The overall goal of this proposal is to perform experiments to discover this molecule, which mediates GMN-AAV brain targeting and transduction. Our hypothesis is that the receptor is a plasma membrane-associated protein or glycan moiety expressed on the surface of brain vascular endothelial cells in TPP1 deficient brain. In aim one, we will use affinity purification and quantitative mass spectrometry methods to isolate and identify the GMN-AAV receptor. In aim two, we will use mass spectrometry glycan profiling, and a mammalian glycan array developed by the NIH-operated Consortium for Functional Glycomics to determine whether GMN-AAV binds to specific glycans or classes of glycans that may be present on the surface of brain endothelial cells. Completion of these aims will enable us to advance the translation of this novel gene therapy to children with LINCL. In addition, this therapeutic strategy could be broadly applied to other LSDs that cause central nervous system dysfunction and neurodegeneration.